The present invention relates generally to the processing of clay and, more particularly, to an improved method for concentrating a beneficiated aqueous kaolin clay slurry which includes evaporating water therefrom through spray drying wherein at least a portion of the energy expended in evaporating water from the slurry is recovered and used in the clay processing.
Kaolin clay has many known applications in industry, including use as a filler in papermaking, a coating for paper, and a pigment in paint. However, crude kaolin clay typically contains various impurities which cause discoloration. Additionally, crude kaolin is typically too abrasive for direct use in these products. Therefore, it is necessary to beneficiate the crude kaolin clay by various well-known commercial processes which increase the brightness of the kaolin by removing discoloration impurities and decrease the abrasiveness by reducing the particle size of the kaolin particles.
In general, such processes for beneficiating crude kaolin clay require that the clay be processed as a low solids slurry. Therefore, it is necessary to add substantial amounts of water to the dry crude kaolin clay to form a clay suspension or slurry having a low solids content, typically in the range of 15% to 40% by weight. However, for commercial applications, the beneficiated clay slurry must have a much higher solids content. Typically beneficiated kaolin clays are shipped commercially for use in paper making, paper coating and paint making as a high solids slurry having a solids content in the range of 65% to 75% by weight. Therefore, most of the water added to the dry kaolin clay must be removed in order to concentrate the clay solids.
To dewater a beneficiated clay slurry, the low-solids slurry is typically first passed to a vacuum or press type filter wherein a limited portion of the water is removed from the slurry. Typically, the filter cake from the filter would have a solids content of about 50% to 60% by weight. Thus, the slurry would still comprise about 40% to 50% water. Further dewatering on a vacuum or press type filter is impractical due to the fine particle size of the solids in the beneficiated clay slurry. Typically, to further dewater the beneficiated clay slurry to a commercially acceptable solids content, at least a portion of the partially dewatered slurry is passed through a spray dryer or other direct contact-type evaporator such as a gas-fired kiln, wherein the clay slurry is contacted with a drying medium having a temperature of 1000.degree. F. or more, such as hot air or hot flue gas typically generated from the combustion of natural gas. Although all of the clay slurry may be passed through the spray dryer for drying, it is customary to pass only a portion of the clay slurry through the spray dryer and then to re-mix the thoroughly dried clay slurry from the spray dryer with the remaining portion of partially dewatered slurry in a high shear mixer to produce a product clay slurry having a solids content of 65% to 75%.
A problem associated with concentrating clay slurries in spray dryers is that spray drying is a relatively inefficient process and considerable energy must be consumed in the spray drying process in order to evaporate the water in the clay slurry. In prior art spray drying systems used for concentrating clay slurries, the water vapor evaporated from the clay slurry in the drying process is typically vented to the atmosphere in the exhaust gas from the dryer. Thus, the energy expended in evaporating the water vapor is wasted.
It is therefore a general object of the present invention to provide an improved method for concentrating a beneficiated aqueous clay slurry via spray drying in an energy efficient manner wherein the energy expended in evaporating water from the clay slurry is at least in part recovered and used in the clay slurry processing.